Metals have useful properties that are unique to each type of metal. Each of these types of metals are used either alone or in the form of an alloy for surface treatment, coating, demagnetizing and decorating of electronic materials, electronic devices, mechanical materials and so forth, as well as catalysis, sterilization, and even in the fields of pharmaceuticals and agricultural chemicals etc. by utilizing their respective characteristic performance (such as electrical conductivity, resistance, semi-conductivity, transparency, ionicity, corrosion resistivity, friction, light blocking, coloring and/or metallic luster) after forming into a film by various means on a substrate such as ceramics and so forth.
The following lists examples of technologies of the prior art used to produce metal films: (1) methods that require a high vacuum such as sputtering; and, (2) methods in which metallic ink is applied and baked such as the thick film paste method. The sputtering method of (1) requires an expensive, high-vacuum and large apparatus, has poor mass-productivity due to batch production, and has high production costs. Consequently, the paste method of (2) is used because of its low equipment cost and high productivity. In this method, various types of metal paste are coated onto a substrate and baked, allowing metal films to be produced continuously and inexpensively using a simple process and apparatus.
The paste used in this thick film paste method is a heterogeneous viscous liquid simply comprising forming various types of metal into fine particles and dispersing in a solvent. Coating and baking this viscous liquid results in a metal film in which the metal particles simply make contact, thereby preventing the formation of a uniform film. Consequently, in the case of silver-palladium alloy that is most frequently used in the electronics industry, even if formed with the thick film paste method, it is necessary to bake by heating to a high temperature of about 950.degree. C. and physically melt the metal particles to obtain a uniform thin film. Consequently, only ceramic substrates, metal substrates or other substrates having a high melting point could be used for the substrate on which the metal film is formed. In addition, a large baking oven and peripheral facilities able to withstand high temperatures, and an energy source and so forth for high-temperature baking are required to perform baking at high temperatures.
Consequently, if the paste baking temperature could be lowered further, equipment costs could be reduced, energy could be saved, and costs could be lowered. In addition, since it desirable to form a metal film on a general-purpose, inexpensive substrate having a low softening point (such as glass or plastic etc.), it is preferable to further lower the baking temperature at which metal film is formed to allow use of those substrates.
In order to overcome this shortcoming, a method has been proposed (organometallic (MO) method) in which inorganic metal particles are first converted to an organometallic compound, coated after uniformly dissolving in a solvent, and then baked to obtain a thin, uniform metal film. For example, in the case of gold, a paste method using an organometallic compound containing sulfur allows the formation of a uniform gold film demonstrating equal performance while requiring approximately only 1/7 the amount of gold as compared with the thick film paste method in which fine particles of gold are kneaded into a paste. Consequently, in fields that use gold, production is switching over to the MO method from the conventional thick film paste method.
A synthesis method in which an organic substance is bonded directly to a metal to transform into an organometallic compound is typically used as a method for producing the above-mentioned organometallic compound. In this synthesis method, a special production method is required to convert inorganic metal into an organometallic compound. In addition, since it is difficult to convert at a yield of 100% at that time, this method is more expensive than methods using metal and solvent. Thus, although this method can be used practically in the case the base metal itself is an expensive metal like gold, in the case of other metals, the use of this method results in the cost required for the process of converting to an organometallic compound being higher than the price of the metal itself. Consequently, there is a need in industry for a metal paste that allows metal films to be produced both inexpensively and easily.